The present invention relates to systems and methods for electrochemical reactions and more particularly to catalysis in heterogeneous media containing a dispersed solid and a liquid phase of low ionic strength and in the presence of a low electrifying force.
In general, chemical reactions can by enhanced by manipulation of relevant local environmental conditions. The rate of the reaction might increase. This might also increase selectivity. Reactions which would otherwise have low yield or be prohibitively expensive might thus be commercially feasible.
Catalysts and enzymes substantially increase the rate of a reaction even if present in small concentrations. The mechanism for this enhancement is usually expressed in terms of reducing the activation energy of the reaction. Not all chemical reactions can be so enhanced and they are often enhanced only under a limited set of conditions.
Reactions may be enhanced by increasing the temperature or pressure. The mechanism for this enhancement is usually expressed in terms of increasing the likelihood of overcoming the activation energy. However, such enhancement may have undesirable aspects. For example, ecologically dangerous but highly stable polychlorinated biphenyls (PCBs) may be destroyed or detoxified by incineration at temperatures of between 800.degree. and 3000.degree. C. For such an operation, the energy costs are high and the gas and solid slag waste may still be environmentally unsafe.
Externally applied electric fields affect physical processes in electrorheological fluids, such as slurries, and are used in electrophoresis and field-flow fractionation to separate phases. Reaction rates of many chemical processes are known to be affected by the presence of an electric field, as in Friedel-Crafts, decomposition, proton-transfer reactions, and field-induced effects at surfaces. However, these applications all involve high electric field strengths of at least 1000 V/cm or even as high as several V/.ANG.. It is undesirable to use such high fields because they might result in unwanted ionization (such as hydrolysis) or cause unwanted reactions to occur.
In electrolysis, electron transfer is a critical reaction step. Electrons are provided or removed at appropriate electrodes. Conventional electrolysis is typically carried out in media with high ionic strength, usually provided by electrolytic solutions or molten salts and with low applied voltages, typically under 2 volts. The concentration of ions and salts might be higher than that of reactants, thereby limiting desired reaction paths (or providing additional unwanted reaction paths). Furthermore, the limited voltage window in electrolysis due to the high ion and salt concentration could block desired reaction paths which might prefer larger voltage fields. In other words, certain reactions are unreachable with conventional electrolysis.
In dispersion electrolysis, metal spheres or supported-metal particles are suspended in a high-impedance medium between feeder electrodes. Due to the small size of the metal spheres and supported metal clusters, the unique properties of microelectrodes apply--electrolysis of small amounts of material in the absence of supporting electrolyte salt. However, the suspension provides for a large number of particles so that the resulting macroscopic electrode area is large; this makes it possible to electrolyze relatively large quantities of material at the ensemble of microelectrodes. Since dispersion electrolysis is a form of electrolysis, electron transfer is a critical reaction step. Dispersion electrolysis has thus far not been demonstrated for reactions other than water decomposition, hydrogen oxidation, and oxygen reduction.